The commission said the animal rights organisation's claim, published in a press release last year, had contravened charity rules on party political neutrality.

The league's claim was based on the results of a YouGov poll it commissioned asking whether the Conservatives' pledge to hold a vote on hunting was "more in keeping with a nasty party or a compassionate Conservative party".

More than twice as many respondents described the Conservatives as ‘nasty' than ‘compassionate', the press release said.

In a regulatory case report published today, the commission said it was "concerned about the wording of the poll", which "appeared to be designed to elicit a particular response for the purpose of criticising the party".

It said: "The wording chosen by the charity was party political in character and went beyond the sort of statement that a charity can properly make."

Douglas Batchelor, chief executive of the league, said it accepted the commission's findings.

But he said he was encouraged that the commission had accepted that the league's Keep Cruelty History campaign, which reveals where election candidates stand on the repeal of the Hunting Act, was a legitimate charitable activity.

Angela Smith, the Minister for the Third Sector, was head of political and public relations at the LACS from 1995 to 1997. Chris Williamson, a current trustee, is standing for Labour in Derby North.

Steve Taylor, head of campaigns at the LACS, said that there was a perception that the organisation, which became a charity in 2003, was close to Labour, but the reality was different.

"We have close relationships with some Tory and Liberal Democrat MPs as well as Labour ones," he said.

"I don't think the perception that we are Labour supporters exists in Westminster; I think it exists outside Westminster."

Nick Hurd, shadow third sector minister, said: "The commission has drawn the right conclusion. This was not appropriate or acceptable behaviour and I would say the same if the target was the Labour Party."